1. Field of the Invention
The present invention relates generally to cryosurgical devices. More particularly, the invention concerns a surgical apparatus that has the ability to control the freezing process both in space and in time by means of a cryoprobe having a plurality of removable cryotips, and a compensating temperature control system operably associated with the cryoprobe.
2. Discussion of Prior Art
Cryosurgery is a surgical procedure that uses freezing temperatures to destroy tissue. James Arnott, an English physician, was the first to introduce this method in 1865 for treatment of cancer of the skin. Between 1920 and 1940, the commercialization of liquid air led a number of surgeons to employ freezing to accomplish the destruction of nondesirable tissue. By 1930 the first monograph on the method was published (Lortat-Jacobs and Solente, 1930).
Modern cryosurgery started with the work of a New York surgeon, I. Cooper, who in 1961 developed a new apparatus for cryosurgery. This apparatus consisted of a hollow metal tube which was vacuum insulated, except at the tip, through which liquid nitrogen was circulated. Cooper was able to localize the freezing and, thereby, treat the tissue in a controlled way. The method was used first for treatment of Parkinsonism, and later extended to the destruction of non desirable tissue in other areas, such as dermatology, proctology, gynecology. The applications of cryosurgery are numerous and have been described in several texts and review papers, (Rand et al., 1968; Ablin 1980; Gage 1982; Zacarian, 1985; Gage, 1988; Gage and Torre, 1988; Onvk and Rubinsky, 1988).
Until recently there were two major problems that hindered the efficient application of cryosurgery to the treatment of cancer and other nondesirable tissue. First, it was impossible to observe the extent of the frozen region during cryosurgery, and second there was no good understanding of the mechanism by which tissue is destroyed during freezing. Consequently, cryosurgery was typically used for treatment of disease in easily accessible areas where the extent of the frozen tissue could be observed visually. Furthermore, since the process of freezing was associated with damage to tissue, it was assumed that the lower the temperature to which the tissue is frozen, the greater the chances for destruction of the tissue. Therefore, the standard prior art approach to cryosurgery was to expose the tissue to as low a temperature as possible. More particularly, it was assumed that lowering the temperature of the tissue to -50 degrees C. would ensure the destruction of the tissue. The existing devices for cryosurgery reflect this particular state of knowledge.
The prior art devices are, in general, of the spray type, wherein the cold refrigerant is sprayed directly onto the tissue to be destroyed, or the closed end cryotip type, in which the refrigerant is delivered to a portion of the tip that is inserted in the tissue to be necrosed. Apparatus described in U.S. Pat. No. 4,376,376 issued to Gregory is exemplary of the spray type devices. The device described in U.S. Pat. No. 4,211,231 is exemplary of the closed end cryotip devices. Typical to all the prior art devices, which were developed in response to the known science at that time, is the fact that the extent of the freezing region is not controlled accurately because there was no way to observe the dimension of the tumor and of the tumors deep in the body. Therefore, an accurate control would not have been useful in any event. Also, the prior art systems were designed to achieve the lowest possible temperature on the tip, as fast as possible, to ensure that as much of the tip as possible is frozen to as low a temperature as possible.
Two major new advances were made recently in the area of cryosurgery. They are reviewed in the paper by Rubinsky and Degg, Proc., R. Soc. Lond. B234, 343-358 (1988). It was found that ultrasound can be used intraoperatively to determine, in real time, the extent of the tumors, as well as that of the frozen tissue during cryosurgery. Ultrasound works by sensing a pressure wave from a pressure transducer. The wave is reflected from boundaries between regions that have differences in acoustic impedance such as between tumors and normal tissue, blood vessels and tissue and frozen and unfrozen tissue. The reflected wave is identified by the pressure transducer and the extent of the tumor, or of the frozen region, is shown on a monitor. Following computerized interpretation of the data, this procedure facilitates an accurate identification of the extent of the tumor and of the frozen region during cryosurgery. Also, recent experiments described in the previously mentioned article by Rubinsky and Degg, have shed new light on the process of freezing in tissue. The results show that freezing in tissue is strongly affected by the structure of the tissue. Ice does not form uniformly throughout the tissue. Rather, it was shown that ice forms first in the blood vessels, while the cells surrounding the frozen blood vessels remain unfrozen. The rejection of saline during the freezing of the blood vessels causes an increase in the saline concentration in the solution inside the blood vessels. This causes water to leave the unfrozen cells through the cell membrane into the blood vessel. The consequent expansion of the blood vessels leads to the destruction of the vessels. Apparently the destruction of the frozen tissue is promoted by the fact that during freezing the vasculature network is destroyed and, therefore, cancerous and other nondesirable cells in the region that has been frozen are deprived of their blood supply after thawing and die because of ischemic necrosis. It was shown in the same paper that tissue can be destroyed by freezing to temperatures as high as -2 degrees C., and that temperatures as low as -50 degrees C. are not required for tissue destruction if the freezing process is done in such a manner as to ensure the destruction of the vasculature network. Destruction of the vasculature network can be achieved by varying the temperature of the cryosurgical tip in a predetermined controlled way. It is this aspect of cryosurgery to which the apparatus of the present invention is directed.
To summarize the new developments: (a) it is now possible to identify accurately both the tumor deep in the body and to observe, in real time, the extent of the frozen region; and (b) it has been established that a more efficient method of cryosurgery can be achieved by varying the temperature of the cryosurgical probe in a controlled way to ensure the destruction of tissue. These new developments can be compared to the previous state of the art in which, (a) it was impossible to determine accurately the tumor and the frozen region deep in the tissue; and (b) it was taught that to insure the destruction of the tissue, it is necessary to cool the tissue as fast as possible and to temperatures as low as possible. Consequently, the previous cryosurgical devices were designed in such a way that, (a) no importance was given to the exact extent of the frozen region because there was no way to know how much tissue is frozen during cryosurgery, or whether the whole tumor was frozen; and (b) the device was designed to deliver as much cooling power as possible locally to freeze the tissue to a temperature as low as possible without any concern being given to control over the temperature history during freezing.